Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing exhaust emissions. For example, natural gas has been recognized as an attractive alternative fuel. For automotive applications, natural gas may be compressed and stored as a gas in cylinders at high pressure. A pressure regulator may then be used to supply the compressed natural gas (CNG) at lower pressures to an engine combustion chamber. The pressure regulator may provide this gaseous fuel at a fixed, constant pressure to the engine, or it may be a variable pressure regulator which can provide gaseous fuel at varying pressures to the engine.
Many advantages may be achieved by using a variable pressure regulator to provide gaseous fuel to the engine, instead of a pressure regulator which provides gaseous fuel to the engine at a fixed, constant pressure. For example, varying the pressure of gaseous fuel increases the dynamic range of the injector (the injector's dynamic range being the “turn down ratio” of fuel that can be injected). Providing a lower fuel pressure during low fuel demand allows for a longer fuel injection pulse width, which allows for injections of lower, repeatable mass. As another example, varying the pressure of gaseous fuel enables use of a lower pressure of gaseous fuel during engine cold start when only a marginal voltage is available to open the injectors, because the lower injector opening voltage may be sufficient for injecting gaseous fuel at a lower injection pressure. As still another example, varying the pressure may allow rare, peak fuel demands to be satisfied without having to subject the injector to the durability challenge of injecting high pressure gaseous fuel at all times.
Despite the advantages of variable pressure regulators, known variable pressure regulators are costly and prone to instability. For example, in some systems, variable pressure regulation is achieved by exposing the reference chamber of the regulator to intake manifold pressure. However, this dependency on intake manifold pressure limits the operability of the pressure regulator when intake manifold vacuum is not within a certain range. In other systems, pressure variability is achieved by changing the reference pressure via a valve from the high pressure source. As another example, one known variable pressure regulator varies the pressure of gaseous fuel by duty cycling a main valve between the regulator and the fuel rail. However, systems which rely on a single valve to perform pressure regulation, where the valve is subject to a flow of high pressure gaseous fuel, may not be adequately robust. For these and other reasons, vehicles may include mechanical pressure regulators which provide a fixed, constant pressure of gaseous fuel to the engine.
In contrast to the variable pressure regulation systems described above, the inventors herein have recognized that gaseous fuel may be provided to the engine at varying pressures in a cost-effective manner, without compromising stability, by raising and lowering the regulating pressure (via raising and lowering the pressure in the reference chamber) of a mechanical pressure regulator. A passage coupling a lower pressure chamber of the regulator with the reference chamber may include a valve, which may be controlled to flow gaseous fuel from the high pressure chamber to the reference chamber to increase the pressure of the reference chamber. Further, a passage coupling the reference chamber to the engine (e.g., to the intake manifold, crankcase, ejector vacuum, vacuum pump vacuum, or fuel vapor storage canister) or to atmosphere may include another valve to reduce the pressure of the reference chamber. Via control of these valves, the pressure in the reference chamber may be varied, which in turn varies the pressure of the gaseous fuel delivered to the engine (e.g., to fuel injectors of the engine) from the low pressure chamber of the regulator. The valves may be controlled by an electronic pressure feedback system, and may be fixed orifice or powertrain control module (PCM) controlled solenoid valves. Superimposing an electronic pressure feedback control system on top of a mechanical pressure regulation system in this way enables variation of the pressure of gaseous fuel delivered to the engine via control of these valves. Advantageously, the valves may be small and inexpensive, and yet the system may still outperform variable pressure regulation approaches which involve duty cycling a main valve between the regulator and the fuel rail.
In one example, a method for regulating gaseous fuel pressure in an engine comprises varying a regulating pressure in a low pressure chamber of a pressure regulator by controllably flowing gaseous fuel into and/or out of a reference chamber of the pressure regulator. This method may enable a mechanical pressure regulator to regulate fuel pressure to different pressures, via the flow of fuel into or out of the reference chamber. For example, the regulating pressure may be increased by controlling a pressure up valve to flow gaseous fuel into the reference chamber from the low pressure chamber, and the regulating pressure may be decreased by controlling a pressure down valve to flow gaseous fuel out of the reference chamber. The engine fuel rail may communicate with the low pressure chamber of the regulator, and thus the regulating pressure may be a pressure which results in a desired fuel rail pressure being achieved. The valves may be controlled based on electronic pressure feedback, e.g. feedback a fuel rail pressure sensor indicating a current fuel rail pressure, and based on engine operating conditions. For example, during low load conditions it may be desirable to inject fuel at a lower pressure relative to a current fuel rail pressure.
In this way, variable pressure regulation may be achieved by controlling valves to flow relatively small amounts of gaseous fuel into and out of the reference chamber of a pressure regulator (as compared to the amount of gaseous fuel flowing from the regulator to the fuel rail), rather than by (or in addition to) duty cycling a main valve in a conduit delivering fuel to the fuel rail. As such, smaller and lower cost valves may be used in the claimed configurations, relative to a system utilizing the duty cycling approach, which results in cost savings, if desired. Further, the gaseous fuel exhausted from the reference chamber when the pressure down valve is opened may be directed to the engine for combustion, thereby improving fuel efficiency. Furthermore, in embodiments where actuators of the pressure up and down valves are PCM controlled, the PCM has knowledge of the extra fuel flow to the engine and can compensate appropriately.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.